Ultrafast low-energy electron diffraction in transmission resolves polymer/graphene superstructure dynamics Max Gulde et al. Science 345, 200 (2014); DOI: 10.1126/science.1250658
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Data reported herein are available in the supplementary materials. Support from the U.S. Department of Energy Office of Basic Energy
Ultrafast low-energy electron diffraction in transmission resolves polymer/ graphene superstructure dynamics Max Gulde,1 Simon Schweda,1 Gero Storeck,1 Manisankar Maiti,1 Hak Ki Yu,2 Alec M. Wodtke,2,3 Sascha Schäfer,1 Claus Ropers1* Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Ultrafast structural probing of such systems offers direct time-domain information on internal interactions and couplings to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction and investigate in transmission the structural relaxation in a polymer/graphene bilayer system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered to a free-standing graphene substrate. We extract the time scales of energy transfer across the bilayer interface, the loss of superstructure order, and the appearance of an amorphous phase with short-range correlations. The high surface sensitivity makes this experimental approach suitable for numerous problems in ultrafast surface science.
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1 4th Physical Institute, University of Göttingen, 37077 Göttingen, Germany. 2Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany. 3Institute for Physical Chemistry, University of Göttingen, 37077 Göttingen, Germany.
*Corresponding author. E-mail: [email protected]
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www.sciencemag.org/content/345/6193/197/suppl/DC1 Materials and Methods Figs. S1 and S2 Tables S1 to S3 References (26–28)
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he investigation of atomic-scale dynamics with high spatiotemporal resolution yields insights into ultrafast structural reorganizations associated with energy transfer or phase transitions. Substantial progress was made in establishing methods for the timeresolved structural analysis of bulk media, including ultrafast implementations of x-ray crystallography (1–3), high-energy electron diffraction (4–6), and microscopy (7–9), as well as time-resolved x-ray and electron spectroscopy (10, 11). In contrast, structural dynamics at surfaces, interfaces, and ultrathin films remain largely elusive, as the surface signal in both x-ray and high-energy electron diffraction is typically masked by large bulk contributions. This limits our ability to study quasi–two-dimensional (2D) systems exhibiting characteristic phase transitions and topologically controlled ordering (12, 13), as well as the dy-
Science to the SUNCAT Center for Interface Science and Catalysis is gratefully acknowledged. A.J.M. is grateful for support by the U.S. Department of Defense through the National Defense Science and Engineering Graduate Fellowship Program.
namics of surface reconstructions and complex adsorbate superstructures (14, 15). Ultrafast electron scattering in grazing incidence enhances the surface signal (14, 16) but faces particular challenges in quantitative diffraction analysis. Optimal surface sensitivity would be attained with an ultrafast implementation of low-energy electron diffraction (LEED). At electron energies of tens to a few hundreds of electron volts, scattering cross sections are strongly increased, which allows for probing depths of only a few monolayers and has made LEED a widely used tool for surface structure determination. However, at such low energies, it has proven exceedingly difficult to implement pulsed electron sources that fulfill the requirements of an ultrafast diffraction experiment (17–19), that is, short pulse duration and low beam emittance. Laser-triggered electron emission from nanoscale photocathodes (20, 21) is expected to resolve some of these issues (22–24), providing well-collimated low-energy electron pulses and a temporal resolution that is comparable to state-of-the-art ultrafast x-ray or high-energy electron diffraction. Motivated by these prospects, we have undertaken the development of a new diffraction apparatus.
17 March 2014; accepted 2 June 2014 10.1126/science.1253486
We have developed transmission ultrafast LEED (T-ULEED) based on a nanometric needle photocathode and demonstrate its capability to resolve atomic-scale structural dynamics of surfaces and monolayer films with a temporal resolution of a few picoseconds. Specifically, we studied the ultrafast laser-driven dynamics of a polymer superstructure on freestanding monolayer graphene. In the laser-pump/electron-probe scheme (Fig. 1A), the sample is excited out of equilibrium by amplified femtosecond laser pulses (800 nm wavelength, 80-fs pulse duration, repetition rate 10 kHz, focal diameter about 100 mm). To minimize hot electron emission from graphene (25), the pump pulse is temporally stretched to 3 ps by dispersion, which, however, is still sufficiently short to resolve the processes described below. The pump-induced structural dynamics are probed by ultrashort electron pulses emitted from a sharp tungsten tip (50-nm radius of curvature), triggered by the second harmonic of the laser. These electron pulses (up to 100 electrons per pulse) are collimated and focused onto the sample at variable electron energies using an electrostatic lens assembly in a geometry that we have recently studied numerically (22). Scattered electrons are subsequently recorded in a transmission geometry by a phosphor screen microchannel plate detector (MCP, Hamamatsu F2226-24P). With our laser-triggered low-energy electron source, diffraction patterns can also be recorded in backreflection and for a range of electron energies, as demonstrated in the supplementary materials (26) (figs. S6 to S8). The electron pulse duration and the spatial and temporal overlap (delay time, Dt = 0) of the laser-pump and electron-probe pulses are determined via a transient-electric-field effect near a bare transmission electron microscopy (TEM) copper grid. Upon excitation of a single copper grid bar with high peak intensity (fluence up to 30 mJ/cm2, unstretched pump pulses), a dense electron cloud is emitted (25, 27), which may lead to a spatial deflection of the passing electron pulse (Fig. 2A). Projection images of the grid before and after Dt = 0 are shown in Fig. 2B, acquired by defocusing the pulsed electron beam. The central distortion in the lower image indicates the extension of the pump-induced electron cloud. Using a collimated electron beam passing a single mesh sciencemag.org SCIENCE
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cell, the effective temporal resolution of the diffraction experiment can be evaluated. Figure 2C shows the delay-dependent electron signal within a detector segment corresponding to a fewmicron-sized mesh region. The observed transient displays a 2-ps width for 450 eV electron energy. This represents an upper bound for the electron pulse duration when using a partial beam because both plasma-cloud formation and beam traversal times limit the probing speed of the transientelectric-field method at low energies to the order of a picosecond. Averaging over the deflection of the entire collimated beam yields an upper limit of about 6 ps. Using these experimental capabilities, we have studied the dynamics of a freestanding polymer/ graphene bilayer. The relevance of this model system entails at least two aspects. From the perspective of polymer physics, considerable interest exists in thin polymer films on solids and the role of spatial confinement on the dynamics or key characteristics of phase and glass transitions (28, 29). Ultrathin films of monolayer dimensions, on the other hand, will exhibit structural and dynamical features governed by short-range polymersubstrate interactions involving in-plane atomic corrugation (30, 31). Furthermore, increasing efforts are devoted to developing graphene-based heterostructures, with polymers serving both as important compounds in graphene transfer and as interface candidates (32, 33). We prepared an ultrathin atactic poly(methyl methacrylate) (PMMA) layer adsorbed to freestanding monolayer graphene on a TEM grid (26). A projection image of a partially covered grid is shown in Fig. 1C. Local diffraction images from selected sample regions, as shown in Fig. 1B, can be recorded by passing the electron beam through
a single mesh cell. The diffraction pattern displays peaks corresponding to the sixfold symmetry of single crystalline graphene and additional spots closer to the central beam stop. The latter are associated with the polymer adsorbate, which forms a superstructure that is orientationally linked to the graphene substrate. We use cryo-TEM in local diffraction mode to identify the polymer order on the scale of few tens of nanometers, finding a reduced symmetry of the diffraction pattern with only two superstructure peaks (fig. S4). This implies that the polymer chains locally exhibit a stripe-like order, registered to the graphene substrate, and with a well-defined periodicity a = 4.26 Å, doubled relative to that of the graphene lattice (Fig. 1, D and E). The angular correlation with the substrate allows for three equivalent superstructure domains rotated by 60° with respect to each other (Fig. 1F) (denoted by half-integer Miller indices). The stripe-like structure follows from the tendency of the polymer strands to adapt to the periodically corrugated adsorption potential by forming 2D folded-chain crystals on the substrate. Similar behavior has been reported for PMMA adsorption on graphite and mica surfaces (34, 35), even for atactic polymers and with the microscopic structure depending on tacticity (35). In contrast to the ordering perpendicular to the chains (36), the diffraction images show no clear sign for pronounced order of the atactic polymer along the chains. We have found that the polymer superstructure can be reversibly melted by laser excitation. Figure 3 summarizes the changes in the diffraction maps induced by the optical pump. The graphene and superstructure diffraction spots of the bilayer system before and after excitation are shown in Fig. 3A.
A normalized difference image is displayed in Fig. 3B. Whereas no prominent changes are observed for the graphene peaks, the intensity of the superstructure spots substantially decreases. The structural evolution is illustrated in Fig. 3C, showing difference images at three pump-probe delays. These difference maps display two main features, namely a reduction of the superstructure intensity (blue spots) and an increase of diffraction at smaller scattering angles corresponding to an in-plane momentum transfer k|| of less than 1.25 Å−1 (red disc). To differentiate the evolution of the inner disc from that of the superstructure spots, we decompose the diffraction changes into an isotropic and a sixfold symmetric component in Fig. 3, D and E, respectively. Specifically, we compute angular averages of the images in azimuthal segments around and in between the spots, and plot the diffraction changes along the radial direction (fig. S2). The increase of small-angle scattering (Fig. 3D) is peaked around k|| = 1.1 Å−1, which is also discernible as a ring shape in Fig. 3C (central panel). The analysis in Fig. 3, D and E, shows a strong dependence on scattering angle in the time scales of positive and negative signal changes. For example, while the spot intensity decrease is nearly complete after 160 ps (Fig. 3E, blue line), substantial relaxation still occurs in the nearly isotropic small-angle region at later times (Fig. 3D). Figure 4 presents a more quantitative picture of the diffraction changes based on time curves and fluence dependencies. The delay-dependent reduction of the superstructure diffraction intensity, representing the loss of crystalline order, occurs on a time scale of ~105 ps (Fig. 4A, blue triangles). This temporal evolution is accompanied
Fig. 1. Ultrafast low-energy electron diffraction in transmission and polymer/graphene bilayer system. (A) Schematic of the laser-pump/electronprobe setup. (B) Diffraction pattern of polymer/graphene bilayer system (at increased electron energy of 1 keV; integration time 1 s). (C) Projection image of one of the samples (450 eV). Dark areas indicate bilayer coverage. (D) Idealized sketch of stripe-like polymer superstructure domains of two different orientations. (E) Zoom into area denoted in (D). Solid (dashed) arrows indicate lattice vectors for real (reciprocal) space unit cell of graphene. Chain spacing denoted by a. (F) Central part of diffraction pattern indicating Miller indices and reciprocal distance a* = 2p/a = 1.47 Å−1. SCIENCE sciencemag.org
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by an inward shift of the remaining spot intensity (Fig. 4B), which amounts to an increase of the polymer interchain distance of about 0.23 Å or 5%. The concurrent appearance of the small-angle diffraction disc is evaluated in Fig. 4A (red circles, averaged over the entire disc region). We find that the time scale of the diffraction increase is in fact a strong function of the scattering angle, with the slowest dynamics occurring at the smallest momenta, as quantified in the inset of Fig. 4A. We have recorded similar dynamics for multiple samples and observe a distribution of characteristic
superstructure melting-time constants of 95 T 25 ps, which is likely caused by slight variations in sample preparation and storage times. Recrystallization after laser-induced melting of the superstructure occurs on time scales of few tens of microseconds, rendering the repetition rate of 10 kHz suitable for reversible studies. To corroborate the notion of superstructure melting, we have carried out measurements as a function of pump fluence. Figure 4C displays the fluence-dependent change in superstructure peak intensity (green diamonds) relative to the
unperturbed sample, together with the associated structural expansion of the crystalline phase (blue triangles), each evaluated at a delay of 600 ps. Both observables exhibit a threshold behavior at a fluence of 3 mJ/cm2 (dashed line). Beyond this threshold, a strong linear decrease in diffraction intensity is found, and it is accompanied by a kink in the otherwise linear fluence-dependent lattice expansion. These marked changes in the physical properties of the superstructure upon differential energy deposition evidence the qualitative characteristics of a phase transition.
Fig. 2. Temporal characterization of low-energy electron pulses. (A) Schematic of the transient-electric-field effect used for determination of the electron pulse duration. (B) Projection images of bare TEM grid recorded before and after time zero. (C) Delay-dependent intensity change for collimated beam at an electron energy of 450 eV (open circles). Red line, error function fit to transient intensity change. Assuming a Gaussian pulse profile and a step-shaped transient-electric-field response yields an electron pulse duration (full width at half maximum) of about 2 ps.
Fig. 3. Time-resolved diffraction maps of superstructure melting. (A) Diffraction images for negative delay (left) and Dt = 600 ps (right). Pump fluence: 6 mJ/cm2. Electron energy: 450 eV. (B) Difference of images shown in (A) with intensity ln normalized to the superstructure peak intensity I0 = I(Dt < 0) at negative delay times: ln(Dt) = [I(Dt) − l0]/I0. (C) Normalized difference images for central region [dashed rectangle in (B)] for several delay times. (D and E) Relative change of radial diffraction intensity (averaged over azimuthal angles) for isotropic (D) and sixfold symmetric (E) components (26). (The darker region around the bottom superstructure spot arises from reduced MCP quantum efficiency.)
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We interpret the structural dynamics as follows: The initial state of the system is characterized by strongly physisorbed polymer chains within a periodic adsorption potential. The presence of both polymer-substrate and polymer interchain interactions determines the equilibrium state as displaying 2D folded-chain crystallites with three equivalent domains orientationally correlated to the graphene substrate. The widths of superstructure diffraction peaks from various samples indicate typical correlation lengths spanning at least 5 to 10 chain spacings of 4.26 Å each. The strength of the PMMA peaks relative to those of graphene suggests both considerable overall crystallinity and substrate coverage. Because the polymer is optically transparent, the laser excitation primarily deposits energy in the graphene, giving rise to a lattice temperature increase of about 540 K at an incident fluence of 3 mJ/cm2 (26). Whereas the graphene degrees of freedom equilibrate on times comparable to the pulse duration used, coupling to the overlayer is considerably slower. The structural dynamics at fluences below the threshold for loss of crystalline order are valuable to estimate the threshold temperature for superstructure melting and the energy coupling times. In the absence of a phase transition, the energy transfer to the adsorbate causes thermal expansion of up to 0.05 Å. Using the thermal expansion coeffi-
cient of bulk PMMA for a rough estimate yields a threshold temperature of about 165°C (26), close to the melting temperature of the bulk polymer (values ranging from 130° to 160°C). This expansion occurs on a comparatively short time scale of about 43 ps (fig. S1). As the thermal expansion itself should be more rapid given the PMMA sound velocity and the estimated domain sizes, this time constant likely represents the actual energy transfer time across the graphene/polymer interface. Assuming a monolayer polymer film (about 5 Å thickness), this time constant implies thermal Kapitza resistances around 8 × 10−8 m2K/W, which is well in the range of literature values for a polymer/carbon nanotube interface (26). Above the fluence threshold for superstructure melting, qualitatively different structural features are observed, including (i) a loss of polymersubstrate registration, (ii) the appearance of an amorphous phase with enhanced low spatial frequency components, and (iii) an accelerated expansion of remaining crystallites. Notably, these processes occur on time scales longer than the mere layer heating and reflect the intrinsic polymer reorganization (see Table 1): (i) Crossing the threshold, increased thermal fluctuations release a growing fraction of mobile polymer chain segments out of substrate registration, in particular
Fig. 4. Structural dynamics: Time constants and fluence threshold. (A) Delay-dependent superstructure diffraction spot (blue triangles) and disc (red circles) intensities, normalized to negative delay. Solid lines are exponential fits to experimental data. (Inset) Momentum-dependent time constants of small-angle scattering increase. (B) Expansion of crystalline PMMA component. Pump fluence for A and B: 6 mJ/cm2. (C) Fluence-dependent superstructure diffraction intensity (green squares) and expansion (blue triangles), evaluated at Dt = 600 ps. Solid lines, guides to the eye. Dashed line, threshold fluence.
Table 1. Experimentally observed time scales in the PMMA/graphene bilayer system.
Dynamical process Bilayer temperature equilibration Loss of PMMA crystallinity Increase of PMMA chain spacing Structural relaxation of amorphous phase
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Characteristic time (ps) 43 (10) 105 (8) 133 (12) 140–300
at domain boundaries and chain folds. Considering the diffraction peak decrease, this process occurs on a 100-ps time scale. (ii) Subsequently, mobile segments form expanded amorphous structures, which are no longer angle-correlated to the graphene substrate, and contribute to the appearance of the inner diffraction disc. Depending on their spatial frequencies, these rearrangements proceed over durations beyond 300 ps for the range of scattering momenta imaged here (Fig. 4A, inset). The peaked feature in the radial diffraction intensity of the amorphous phase implies a preferred spatial correlation period of 5.7 Å, which is substantially expanded by 25% with respect to the crystalline phase. (iii) The continuous expansion of the crystalline phase above the threshold excludes an interpretation of the melting process based on a gradual exchange between two coexisting thermodynamic phases at a pinned temperature. The sequential relaxation processes observed here are a direct result of the hierarchy of coupling strengths, from the relatively weak interchain interaction over the substrate surface registration to the structural constraint given by the polymer backbone. Presently, it remains an open question whether the enhanced expansion of remaining crystallites above threshold is caused by a loss of crystallinity—e.g., by domain shrinkage—or if itself serves as a major driver of the melting process. The observed time scales are relatively short in comparison with very recently reported intrinsic fluctuations within polymer-graphene heterostructures at similar temperatures (31). This suggests that the loss of superstructure registration involves a release of considerable stress present in the ordered phase. We expect that a FrenkelKontorova–type model and dynamical excitations therein may lead to a detailed microscopic picture (30), provided that the dimensionality of the system and the internal polymer degrees of freedom are considered. The compact experimental implementation of time-resolved LEED is adaptable to conventional ultrafast optics setups and ultrahigh vacuum apparatuses, rendering its application feasible in the broader surface science community. The present operation conditions (energies at the upper end of the LEED range and transmission geometry) were chosen for an optimal combination of low pulse duration and high scattering efficiency providing monolayer sensitivity. Timeresolved LEED measurements in back-reflection from surfaces, including reconstructions and adsorbate layers, will require lower energies than employed here. To demonstrate the suitability of our approach for more general ultrafast surface studies, we provide back-scattering diffraction images and a pulse characterization at lower energy in the supplementary materials (figs. S8 and S5, respectively). We anticipate that further developments toward miniaturized electron guns will in the future allow for femtosecond LEED studies of bulk surfaces at energies of 100 eV and below, opening up the exploration of a whole arena of unconventional phases and transitions in solidstate and molecular surface science. 11 JULY 2014 • VOL 345 ISSUE 6193
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EARTHQUAKE DYNAMICS
Supershear rupture in a Mw 6.7 aftershock of the 2013 Sea of Okhotsk earthquake Zhongwen Zhan,1,2* Donald V. Helmberger,2 Hiroo Kanamori,2 Peter M. Shearer1 Earthquake rupture speeds exceeding the shear-wave velocity have been reported for several shallow strike-slip events. Whether supershear rupture also can occur in deep earthquakes is unclear, because of their enigmatic faulting mechanism. Using empirical Green's functions in both regional and teleseismic waveforms, we observed supershear rupture during the 2013 moment magnitude (Mw) 6.7 deep earthquake beneath the Sea of Okhotsk, an aftershock of the large deep earthquake (Mw 8.3). The Mw 6.7 event ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers per second, suggesting efficient seismic energy generation. Comparing it to the highly dissipative 1994 Mw 8.3 Bolivia earthquake, the two events represent end members of deep earthquakes in terms of energy partitioning and imply that there is more than one rupture mechanism for deep earthquakes.
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ost earthquakes rupture at speeds less than the shear-wave speed (VS). Theory and laboratory experiments indicate that rupture speeds in excess of VS are possible (1–3), and supershear ruptures have now occasionally been reported for large strikeslip events (mode II), including the 1979 Imperial Valley (4), 1999 Izmit (5), 2001 Kunlun (6–8), 2002 Denali (7, 9), 2010 Yushu (10), and 2013 Craig (11) earthquakes. All of these documented occurrences were shallow earthquakes with a simple fault geo1
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093–0225, USA. 2Seismological Laboratory, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA. *Corresponding author. E-mail: [email protected], zwzhan@ gmail.com
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metry (12), and mostly with surface breaks, which is consistent with theoretical studies that the free surface helps promote supershear rupture (13, 14). No definitive evidence has yet been obtained for supershear rupture in deep earthquakes (depth > 300 km) (15). However, the rupture speeds of these events are difficult to estimate because of a general absence of near-field observations, and they appear highly variable. For example, the rupture speeds of the two largest deep earthquakes observed to date, the 1994 moment magnitude (Mw) 8.3 Bolivia earthquake and the 2013 Mw 8.3 Sea of Okhotsk earthquake (16–18), were about 0.2 to 0.4 and 0.7 VS, respectively. About 80% of the rupture velocities for deep earthquakes fall between 0.3 and 0.9 VS (19), a greater range than is seen for shallow earthquakes (15). The rupture speed may depend on the slab ther-
35. J. S. Ha et al., J. Vac. Sci. Technol. B 12, 1977–1980 (1994). 36. Whereas the (10) diffraction spot of PMMA overlaps with that of graphene, the (3/2 0) PMMA spot is not observed, which is most likely a result of the chain form factor or disorder. AC KNOWLED GME NTS
We thank M. Müller for helpful discussions on polymer dynamics. Supporting sample characterizations by H. Schuhmann, M. Seibt, S. Strauch, H. Stark (TEM imaging), S. Dechert, and M. Sivis (Raman spectroscopy), as well as James E. Evans and Nigel D. Browning (high-resolution TEM, Pacific Northwest National Laboratory), are gratefully acknowledged. This work was partially funded by the Deutsche Forschungsgemeinschaft (DFG-ZuK 45/1 and DFG-SFB 1073). M.G. was financially supported by the German National Academic Foundation. A.M.W. and H.K.Y. gratefully acknowledge support from the Alexander von Humboldt Foundation. SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/345/6193/200/suppl/DC1 Materials and Methods Figs. S1 to S8 References (37–48) 9 January 2014; accepted 22 May 2014 10.1126/science.1250658
mal state (20, 21), with ruptures propagating more slowly in warm slabs than in cold slabs, but seismic observations have been inconclusive (22, 23). The one previous example of observed supershear rupture during the 1990 Mw 7.1 Sakhalin deep earthquake neglected to take into account waveform changes from attenuation and the high-velocity subducted slab (24, 25). The 24 May 2013 Mw 8.3 Sea of Okhotsk event (depth, 607 km) was the largest deep earthquake ever recorded (Fig. 1), slightly larger than the 1994 Bolivia earthquake. On the same day, an Mw 6.7 earthquake at a depth of 642 km occurred about 300 km southwest of the mainshock and was recorded by many teleseismic stations and one regional station (Fig. 1). An extraordinary feature of the Mw 6.7 event was its sharp teleseismic P waves, which had displacement pulse widths at most azimuths of 1 to 2 s (Fig. 1). These are much less than the expected source duration of 8 s, based on its magnitude and previous studies of scaled durations of deep earthquakes (26, 27). If taken as a rough estimate of the Mw 6.7 earthquake’s source duration, these very short teleseismic P-wave durations imply extremely high stress drops in a range from 157 MPa to 5.856 GPa (17). On the other hand, the regional station PET (distance ≈ 495 km) on the Kamchatka Peninsula to the east displayed a much longer direct P wavetrain of about 5 s (Fig. 1). Because the P wave to the PET station left the source along an upgoing ray path, instead of the downgoing rays for the teleseismic stations, this longer P-wave duration at PET suggests possible downward rupture directivity during the Mw 6.7 earthquake. However, to test this possibility we first need to account for possible path effects such as wave diffractions along the high-velocity slab in which the earthquake occurred and site effects at the stations. We used waveforms from two nearby smaller earthquakes (Fig. 1; the 24 June 2013 Mw 4.3 sciencemag.org SCIENCE
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7. P. Liao, E. A. Carter, Acta Mater. 58, 5912–5925 (2010). 8. K. Reuter, M. Scheffler, Phys. Rev. B 2, 12 (2001). 9. S. L. Frederiksen, K. W. Jacobsen, K. S. Brown, J. P. Sethna, Phys. Rev. Lett. 93, 165501 (2004). 10. J. J. Mortensen et al., Phys. Rev. Lett. 95, 216401 (2005). 11. J. Wellendorff et al., Phys. Rev. B 85, 235149 (2012). 12. G. Ertl, Catal. Rev. 21, 201–223 (1980). 13. S. R. Bare, D. R. Strongin, G. A. Somorjai, J. Phys. Chem. 90, 4726–4729 (1986). 14. R. Schlögl, Angew. Chem. Int. Ed. 42, 2004–2008 (2003). 15. S. Dahl, P. A. Taylor, E. Törnqvist, I. Chorkendorff, J. Catal. 178, 679–686 (1998). 16. K. Honkala et al., Science 307, 555–558 (2005). 17. Materials and methods are available as supplementary material on Science Online.
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A. Vojvodic et al., Chem. Phys. Lett. 598, 108–112 (2014). S. Dahl et al., Phys. Rev. Lett. 83, 1814–1817 (1999). R. C. Egeberg et al., Surf. Sci. 491, 183–194 (2001). K. Aika, A. Ozaki, in Catalysis: Science and Technology Volume 1, M. Boudart, J. R. Anderson, Eds. (Springer, Berlin, 1981), Chapter 3. A. Logadottir et al., J. Catal. 197, 229–231 (2001). F. Abild-Pedersen et al., Phys. Rev. Lett. 99, 016105 (2007). S. Wang et al., Phys. Chem. Chem. Phys. 13, 20760–20765 (2011). Z. Ulissi, V. Prasad, D. G. Vlachos, J. Catal. 281, 339–344 (2011).
Data reported herein are available in the supplementary materials. Support from the U.S. Department of Energy Office of Basic Energy
Ultrafast low-energy electron diffraction in transmission resolves polymer/ graphene superstructure dynamics Max Gulde,1 Simon Schweda,1 Gero Storeck,1 Manisankar Maiti,1 Hak Ki Yu,2 Alec M. Wodtke,2,3 Sascha Schäfer,1 Claus Ropers1* Two-dimensional systems such as surfaces and molecular monolayers exhibit a multitude of intriguing phases and complex transitions. Ultrafast structural probing of such systems offers direct time-domain information on internal interactions and couplings to a substrate or bulk support. We have developed ultrafast low-energy electron diffraction and investigate in transmission the structural relaxation in a polymer/graphene bilayer system excited out of equilibrium. The laser-pump/electron-probe scheme resolves the ultrafast melting of a polymer superstructure consisting of folded-chain crystals registered to a free-standing graphene substrate. We extract the time scales of energy transfer across the bilayer interface, the loss of superstructure order, and the appearance of an amorphous phase with short-range correlations. The high surface sensitivity makes this experimental approach suitable for numerous problems in ultrafast surface science.
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1 4th Physical Institute, University of Göttingen, 37077 Göttingen, Germany. 2Max Planck Institute for Biophysical Chemistry, 37077 Göttingen, Germany. 3Institute for Physical Chemistry, University of Göttingen, 37077 Göttingen, Germany.
*Corresponding author. E-mail: [email protected]
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www.sciencemag.org/content/345/6193/197/suppl/DC1 Materials and Methods Figs. S1 and S2 Tables S1 to S3 References (26–28)
ACKN OWLED GMEN TS
IMAGING TECHNIQUES
he investigation of atomic-scale dynamics with high spatiotemporal resolution yields insights into ultrafast structural reorganizations associated with energy transfer or phase transitions. Substantial progress was made in establishing methods for the timeresolved structural analysis of bulk media, including ultrafast implementations of x-ray crystallography (1–3), high-energy electron diffraction (4–6), and microscopy (7–9), as well as time-resolved x-ray and electron spectroscopy (10, 11). In contrast, structural dynamics at surfaces, interfaces, and ultrathin films remain largely elusive, as the surface signal in both x-ray and high-energy electron diffraction is typically masked by large bulk contributions. This limits our ability to study quasi–two-dimensional (2D) systems exhibiting characteristic phase transitions and topologically controlled ordering (12, 13), as well as the dy-
Science to the SUNCAT Center for Interface Science and Catalysis is gratefully acknowledged. A.J.M. is grateful for support by the U.S. Department of Defense through the National Defense Science and Engineering Graduate Fellowship Program.
namics of surface reconstructions and complex adsorbate superstructures (14, 15). Ultrafast electron scattering in grazing incidence enhances the surface signal (14, 16) but faces particular challenges in quantitative diffraction analysis. Optimal surface sensitivity would be attained with an ultrafast implementation of low-energy electron diffraction (LEED). At electron energies of tens to a few hundreds of electron volts, scattering cross sections are strongly increased, which allows for probing depths of only a few monolayers and has made LEED a widely used tool for surface structure determination. However, at such low energies, it has proven exceedingly difficult to implement pulsed electron sources that fulfill the requirements of an ultrafast diffraction experiment (17–19), that is, short pulse duration and low beam emittance. Laser-triggered electron emission from nanoscale photocathodes (20, 21) is expected to resolve some of these issues (22–24), providing well-collimated low-energy electron pulses and a temporal resolution that is comparable to state-of-the-art ultrafast x-ray or high-energy electron diffraction. Motivated by these prospects, we have undertaken the development of a new diffraction apparatus.
17 March 2014; accepted 2 June 2014 10.1126/science.1253486
We have developed transmission ultrafast LEED (T-ULEED) based on a nanometric needle photocathode and demonstrate its capability to resolve atomic-scale structural dynamics of surfaces and monolayer films with a temporal resolution of a few picoseconds. Specifically, we studied the ultrafast laser-driven dynamics of a polymer superstructure on freestanding monolayer graphene. In the laser-pump/electron-probe scheme (Fig. 1A), the sample is excited out of equilibrium by amplified femtosecond laser pulses (800 nm wavelength, 80-fs pulse duration, repetition rate 10 kHz, focal diameter about 100 mm). To minimize hot electron emission from graphene (25), the pump pulse is temporally stretched to 3 ps by dispersion, which, however, is still sufficiently short to resolve the processes described below. The pump-induced structural dynamics are probed by ultrashort electron pulses emitted from a sharp tungsten tip (50-nm radius of curvature), triggered by the second harmonic of the laser. These electron pulses (up to 100 electrons per pulse) are collimated and focused onto the sample at variable electron energies using an electrostatic lens assembly in a geometry that we have recently studied numerically (22). Scattered electrons are subsequently recorded in a transmission geometry by a phosphor screen microchannel plate detector (MCP, Hamamatsu F2226-24P). With our laser-triggered low-energy electron source, diffraction patterns can also be recorded in backreflection and for a range of electron energies, as demonstrated in the supplementary materials (26) (figs. S6 to S8). The electron pulse duration and the spatial and temporal overlap (delay time, Dt = 0) of the laser-pump and electron-probe pulses are determined via a transient-electric-field effect near a bare transmission electron microscopy (TEM) copper grid. Upon excitation of a single copper grid bar with high peak intensity (fluence up to 30 mJ/cm2, unstretched pump pulses), a dense electron cloud is emitted (25, 27), which may lead to a spatial deflection of the passing electron pulse (Fig. 2A). Projection images of the grid before and after Dt = 0 are shown in Fig. 2B, acquired by defocusing the pulsed electron beam. The central distortion in the lower image indicates the extension of the pump-induced electron cloud. Using a collimated electron beam passing a single mesh sciencemag.org SCIENCE
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cell, the effective temporal resolution of the diffraction experiment can be evaluated. Figure 2C shows the delay-dependent electron signal within a detector segment corresponding to a fewmicron-sized mesh region. The observed transient displays a 2-ps width for 450 eV electron energy. This represents an upper bound for the electron pulse duration when using a partial beam because both plasma-cloud formation and beam traversal times limit the probing speed of the transientelectric-field method at low energies to the order of a picosecond. Averaging over the deflection of the entire collimated beam yields an upper limit of about 6 ps. Using these experimental capabilities, we have studied the dynamics of a freestanding polymer/ graphene bilayer. The relevance of this model system entails at least two aspects. From the perspective of polymer physics, considerable interest exists in thin polymer films on solids and the role of spatial confinement on the dynamics or key characteristics of phase and glass transitions (28, 29). Ultrathin films of monolayer dimensions, on the other hand, will exhibit structural and dynamical features governed by short-range polymersubstrate interactions involving in-plane atomic corrugation (30, 31). Furthermore, increasing efforts are devoted to developing graphene-based heterostructures, with polymers serving both as important compounds in graphene transfer and as interface candidates (32, 33). We prepared an ultrathin atactic poly(methyl methacrylate) (PMMA) layer adsorbed to freestanding monolayer graphene on a TEM grid (26). A projection image of a partially covered grid is shown in Fig. 1C. Local diffraction images from selected sample regions, as shown in Fig. 1B, can be recorded by passing the electron beam through
a single mesh cell. The diffraction pattern displays peaks corresponding to the sixfold symmetry of single crystalline graphene and additional spots closer to the central beam stop. The latter are associated with the polymer adsorbate, which forms a superstructure that is orientationally linked to the graphene substrate. We use cryo-TEM in local diffraction mode to identify the polymer order on the scale of few tens of nanometers, finding a reduced symmetry of the diffraction pattern with only two superstructure peaks (fig. S4). This implies that the polymer chains locally exhibit a stripe-like order, registered to the graphene substrate, and with a well-defined periodicity a = 4.26 Å, doubled relative to that of the graphene lattice (Fig. 1, D and E). The angular correlation with the substrate allows for three equivalent superstructure domains rotated by 60° with respect to each other (Fig. 1F) (denoted by half-integer Miller indices). The stripe-like structure follows from the tendency of the polymer strands to adapt to the periodically corrugated adsorption potential by forming 2D folded-chain crystals on the substrate. Similar behavior has been reported for PMMA adsorption on graphite and mica surfaces (34, 35), even for atactic polymers and with the microscopic structure depending on tacticity (35). In contrast to the ordering perpendicular to the chains (36), the diffraction images show no clear sign for pronounced order of the atactic polymer along the chains. We have found that the polymer superstructure can be reversibly melted by laser excitation. Figure 3 summarizes the changes in the diffraction maps induced by the optical pump. The graphene and superstructure diffraction spots of the bilayer system before and after excitation are shown in Fig. 3A.
A normalized difference image is displayed in Fig. 3B. Whereas no prominent changes are observed for the graphene peaks, the intensity of the superstructure spots substantially decreases. The structural evolution is illustrated in Fig. 3C, showing difference images at three pump-probe delays. These difference maps display two main features, namely a reduction of the superstructure intensity (blue spots) and an increase of diffraction at smaller scattering angles corresponding to an in-plane momentum transfer k|| of less than 1.25 Å−1 (red disc). To differentiate the evolution of the inner disc from that of the superstructure spots, we decompose the diffraction changes into an isotropic and a sixfold symmetric component in Fig. 3, D and E, respectively. Specifically, we compute angular averages of the images in azimuthal segments around and in between the spots, and plot the diffraction changes along the radial direction (fig. S2). The increase of small-angle scattering (Fig. 3D) is peaked around k|| = 1.1 Å−1, which is also discernible as a ring shape in Fig. 3C (central panel). The analysis in Fig. 3, D and E, shows a strong dependence on scattering angle in the time scales of positive and negative signal changes. For example, while the spot intensity decrease is nearly complete after 160 ps (Fig. 3E, blue line), substantial relaxation still occurs in the nearly isotropic small-angle region at later times (Fig. 3D). Figure 4 presents a more quantitative picture of the diffraction changes based on time curves and fluence dependencies. The delay-dependent reduction of the superstructure diffraction intensity, representing the loss of crystalline order, occurs on a time scale of ~105 ps (Fig. 4A, blue triangles). This temporal evolution is accompanied
Fig. 1. Ultrafast low-energy electron diffraction in transmission and polymer/graphene bilayer system. (A) Schematic of the laser-pump/electronprobe setup. (B) Diffraction pattern of polymer/graphene bilayer system (at increased electron energy of 1 keV; integration time 1 s). (C) Projection image of one of the samples (450 eV). Dark areas indicate bilayer coverage. (D) Idealized sketch of stripe-like polymer superstructure domains of two different orientations. (E) Zoom into area denoted in (D). Solid (dashed) arrows indicate lattice vectors for real (reciprocal) space unit cell of graphene. Chain spacing denoted by a. (F) Central part of diffraction pattern indicating Miller indices and reciprocal distance a* = 2p/a = 1.47 Å−1. SCIENCE sciencemag.org
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by an inward shift of the remaining spot intensity (Fig. 4B), which amounts to an increase of the polymer interchain distance of about 0.23 Å or 5%. The concurrent appearance of the small-angle diffraction disc is evaluated in Fig. 4A (red circles, averaged over the entire disc region). We find that the time scale of the diffraction increase is in fact a strong function of the scattering angle, with the slowest dynamics occurring at the smallest momenta, as quantified in the inset of Fig. 4A. We have recorded similar dynamics for multiple samples and observe a distribution of characteristic
superstructure melting-time constants of 95 T 25 ps, which is likely caused by slight variations in sample preparation and storage times. Recrystallization after laser-induced melting of the superstructure occurs on time scales of few tens of microseconds, rendering the repetition rate of 10 kHz suitable for reversible studies. To corroborate the notion of superstructure melting, we have carried out measurements as a function of pump fluence. Figure 4C displays the fluence-dependent change in superstructure peak intensity (green diamonds) relative to the
unperturbed sample, together with the associated structural expansion of the crystalline phase (blue triangles), each evaluated at a delay of 600 ps. Both observables exhibit a threshold behavior at a fluence of 3 mJ/cm2 (dashed line). Beyond this threshold, a strong linear decrease in diffraction intensity is found, and it is accompanied by a kink in the otherwise linear fluence-dependent lattice expansion. These marked changes in the physical properties of the superstructure upon differential energy deposition evidence the qualitative characteristics of a phase transition.
Fig. 2. Temporal characterization of low-energy electron pulses. (A) Schematic of the transient-electric-field effect used for determination of the electron pulse duration. (B) Projection images of bare TEM grid recorded before and after time zero. (C) Delay-dependent intensity change for collimated beam at an electron energy of 450 eV (open circles). Red line, error function fit to transient intensity change. Assuming a Gaussian pulse profile and a step-shaped transient-electric-field response yields an electron pulse duration (full width at half maximum) of about 2 ps.
Fig. 3. Time-resolved diffraction maps of superstructure melting. (A) Diffraction images for negative delay (left) and Dt = 600 ps (right). Pump fluence: 6 mJ/cm2. Electron energy: 450 eV. (B) Difference of images shown in (A) with intensity ln normalized to the superstructure peak intensity I0 = I(Dt < 0) at negative delay times: ln(Dt) = [I(Dt) − l0]/I0. (C) Normalized difference images for central region [dashed rectangle in (B)] for several delay times. (D and E) Relative change of radial diffraction intensity (averaged over azimuthal angles) for isotropic (D) and sixfold symmetric (E) components (26). (The darker region around the bottom superstructure spot arises from reduced MCP quantum efficiency.)
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We interpret the structural dynamics as follows: The initial state of the system is characterized by strongly physisorbed polymer chains within a periodic adsorption potential. The presence of both polymer-substrate and polymer interchain interactions determines the equilibrium state as displaying 2D folded-chain crystallites with three equivalent domains orientationally correlated to the graphene substrate. The widths of superstructure diffraction peaks from various samples indicate typical correlation lengths spanning at least 5 to 10 chain spacings of 4.26 Å each. The strength of the PMMA peaks relative to those of graphene suggests both considerable overall crystallinity and substrate coverage. Because the polymer is optically transparent, the laser excitation primarily deposits energy in the graphene, giving rise to a lattice temperature increase of about 540 K at an incident fluence of 3 mJ/cm2 (26). Whereas the graphene degrees of freedom equilibrate on times comparable to the pulse duration used, coupling to the overlayer is considerably slower. The structural dynamics at fluences below the threshold for loss of crystalline order are valuable to estimate the threshold temperature for superstructure melting and the energy coupling times. In the absence of a phase transition, the energy transfer to the adsorbate causes thermal expansion of up to 0.05 Å. Using the thermal expansion coeffi-
cient of bulk PMMA for a rough estimate yields a threshold temperature of about 165°C (26), close to the melting temperature of the bulk polymer (values ranging from 130° to 160°C). This expansion occurs on a comparatively short time scale of about 43 ps (fig. S1). As the thermal expansion itself should be more rapid given the PMMA sound velocity and the estimated domain sizes, this time constant likely represents the actual energy transfer time across the graphene/polymer interface. Assuming a monolayer polymer film (about 5 Å thickness), this time constant implies thermal Kapitza resistances around 8 × 10−8 m2K/W, which is well in the range of literature values for a polymer/carbon nanotube interface (26). Above the fluence threshold for superstructure melting, qualitatively different structural features are observed, including (i) a loss of polymersubstrate registration, (ii) the appearance of an amorphous phase with enhanced low spatial frequency components, and (iii) an accelerated expansion of remaining crystallites. Notably, these processes occur on time scales longer than the mere layer heating and reflect the intrinsic polymer reorganization (see Table 1): (i) Crossing the threshold, increased thermal fluctuations release a growing fraction of mobile polymer chain segments out of substrate registration, in particular
Fig. 4. Structural dynamics: Time constants and fluence threshold. (A) Delay-dependent superstructure diffraction spot (blue triangles) and disc (red circles) intensities, normalized to negative delay. Solid lines are exponential fits to experimental data. (Inset) Momentum-dependent time constants of small-angle scattering increase. (B) Expansion of crystalline PMMA component. Pump fluence for A and B: 6 mJ/cm2. (C) Fluence-dependent superstructure diffraction intensity (green squares) and expansion (blue triangles), evaluated at Dt = 600 ps. Solid lines, guides to the eye. Dashed line, threshold fluence.
Table 1. Experimentally observed time scales in the PMMA/graphene bilayer system.
Dynamical process Bilayer temperature equilibration Loss of PMMA crystallinity Increase of PMMA chain spacing Structural relaxation of amorphous phase
SCIENCE sciencemag.org
Characteristic time (ps) 43 (10) 105 (8) 133 (12) 140–300
at domain boundaries and chain folds. Considering the diffraction peak decrease, this process occurs on a 100-ps time scale. (ii) Subsequently, mobile segments form expanded amorphous structures, which are no longer angle-correlated to the graphene substrate, and contribute to the appearance of the inner diffraction disc. Depending on their spatial frequencies, these rearrangements proceed over durations beyond 300 ps for the range of scattering momenta imaged here (Fig. 4A, inset). The peaked feature in the radial diffraction intensity of the amorphous phase implies a preferred spatial correlation period of 5.7 Å, which is substantially expanded by 25% with respect to the crystalline phase. (iii) The continuous expansion of the crystalline phase above the threshold excludes an interpretation of the melting process based on a gradual exchange between two coexisting thermodynamic phases at a pinned temperature. The sequential relaxation processes observed here are a direct result of the hierarchy of coupling strengths, from the relatively weak interchain interaction over the substrate surface registration to the structural constraint given by the polymer backbone. Presently, it remains an open question whether the enhanced expansion of remaining crystallites above threshold is caused by a loss of crystallinity—e.g., by domain shrinkage—or if itself serves as a major driver of the melting process. The observed time scales are relatively short in comparison with very recently reported intrinsic fluctuations within polymer-graphene heterostructures at similar temperatures (31). This suggests that the loss of superstructure registration involves a release of considerable stress present in the ordered phase. We expect that a FrenkelKontorova–type model and dynamical excitations therein may lead to a detailed microscopic picture (30), provided that the dimensionality of the system and the internal polymer degrees of freedom are considered. The compact experimental implementation of time-resolved LEED is adaptable to conventional ultrafast optics setups and ultrahigh vacuum apparatuses, rendering its application feasible in the broader surface science community. The present operation conditions (energies at the upper end of the LEED range and transmission geometry) were chosen for an optimal combination of low pulse duration and high scattering efficiency providing monolayer sensitivity. Timeresolved LEED measurements in back-reflection from surfaces, including reconstructions and adsorbate layers, will require lower energies than employed here. To demonstrate the suitability of our approach for more general ultrafast surface studies, we provide back-scattering diffraction images and a pulse characterization at lower energy in the supplementary materials (figs. S8 and S5, respectively). We anticipate that further developments toward miniaturized electron guns will in the future allow for femtosecond LEED studies of bulk surfaces at energies of 100 eV and below, opening up the exploration of a whole arena of unconventional phases and transitions in solidstate and molecular surface science. 11 JULY 2014 • VOL 345 ISSUE 6193
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EARTHQUAKE DYNAMICS
Supershear rupture in a Mw 6.7 aftershock of the 2013 Sea of Okhotsk earthquake Zhongwen Zhan,1,2* Donald V. Helmberger,2 Hiroo Kanamori,2 Peter M. Shearer1 Earthquake rupture speeds exceeding the shear-wave velocity have been reported for several shallow strike-slip events. Whether supershear rupture also can occur in deep earthquakes is unclear, because of their enigmatic faulting mechanism. Using empirical Green's functions in both regional and teleseismic waveforms, we observed supershear rupture during the 2013 moment magnitude (Mw) 6.7 deep earthquake beneath the Sea of Okhotsk, an aftershock of the large deep earthquake (Mw 8.3). The Mw 6.7 event ruptured downward along a steeply dipping fault plane at an average speed of 8 kilometers per second, suggesting efficient seismic energy generation. Comparing it to the highly dissipative 1994 Mw 8.3 Bolivia earthquake, the two events represent end members of deep earthquakes in terms of energy partitioning and imply that there is more than one rupture mechanism for deep earthquakes.
M
ost earthquakes rupture at speeds less than the shear-wave speed (VS). Theory and laboratory experiments indicate that rupture speeds in excess of VS are possible (1–3), and supershear ruptures have now occasionally been reported for large strikeslip events (mode II), including the 1979 Imperial Valley (4), 1999 Izmit (5), 2001 Kunlun (6–8), 2002 Denali (7, 9), 2010 Yushu (10), and 2013 Craig (11) earthquakes. All of these documented occurrences were shallow earthquakes with a simple fault geo1
Institute of Geophysics and Planetary Physics, Scripps Institution of Oceanography, University of California, San Diego, La Jolla, CA 92093–0225, USA. 2Seismological Laboratory, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA. *Corresponding author. E-mail: [email protected], zwzhan@ gmail.com
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metry (12), and mostly with surface breaks, which is consistent with theoretical studies that the free surface helps promote supershear rupture (13, 14). No definitive evidence has yet been obtained for supershear rupture in deep earthquakes (depth > 300 km) (15). However, the rupture speeds of these events are difficult to estimate because of a general absence of near-field observations, and they appear highly variable. For example, the rupture speeds of the two largest deep earthquakes observed to date, the 1994 moment magnitude (Mw) 8.3 Bolivia earthquake and the 2013 Mw 8.3 Sea of Okhotsk earthquake (16–18), were about 0.2 to 0.4 and 0.7 VS, respectively. About 80% of the rupture velocities for deep earthquakes fall between 0.3 and 0.9 VS (19), a greater range than is seen for shallow earthquakes (15). The rupture speed may depend on the slab ther-
35. J. S. Ha et al., J. Vac. Sci. Technol. B 12, 1977–1980 (1994). 36. Whereas the (10) diffraction spot of PMMA overlaps with that of graphene, the (3/2 0) PMMA spot is not observed, which is most likely a result of the chain form factor or disorder. AC KNOWLED GME NTS
We thank M. Müller for helpful discussions on polymer dynamics. Supporting sample characterizations by H. Schuhmann, M. Seibt, S. Strauch, H. Stark (TEM imaging), S. Dechert, and M. Sivis (Raman spectroscopy), as well as James E. Evans and Nigel D. Browning (high-resolution TEM, Pacific Northwest National Laboratory), are gratefully acknowledged. This work was partially funded by the Deutsche Forschungsgemeinschaft (DFG-ZuK 45/1 and DFG-SFB 1073). M.G. was financially supported by the German National Academic Foundation. A.M.W. and H.K.Y. gratefully acknowledge support from the Alexander von Humboldt Foundation. SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/345/6193/200/suppl/DC1 Materials and Methods Figs. S1 to S8 References (37–48) 9 January 2014; accepted 22 May 2014 10.1126/science.1250658
mal state (20, 21), with ruptures propagating more slowly in warm slabs than in cold slabs, but seismic observations have been inconclusive (22, 23). The one previous example of observed supershear rupture during the 1990 Mw 7.1 Sakhalin deep earthquake neglected to take into account waveform changes from attenuation and the high-velocity subducted slab (24, 25). The 24 May 2013 Mw 8.3 Sea of Okhotsk event (depth, 607 km) was the largest deep earthquake ever recorded (Fig. 1), slightly larger than the 1994 Bolivia earthquake. On the same day, an Mw 6.7 earthquake at a depth of 642 km occurred about 300 km southwest of the mainshock and was recorded by many teleseismic stations and one regional station (Fig. 1). An extraordinary feature of the Mw 6.7 event was its sharp teleseismic P waves, which had displacement pulse widths at most azimuths of 1 to 2 s (Fig. 1). These are much less than the expected source duration of 8 s, based on its magnitude and previous studies of scaled durations of deep earthquakes (26, 27). If taken as a rough estimate of the Mw 6.7 earthquake’s source duration, these very short teleseismic P-wave durations imply extremely high stress drops in a range from 157 MPa to 5.856 GPa (17). On the other hand, the regional station PET (distance ≈ 495 km) on the Kamchatka Peninsula to the east displayed a much longer direct P wavetrain of about 5 s (Fig. 1). Because the P wave to the PET station left the source along an upgoing ray path, instead of the downgoing rays for the teleseismic stations, this longer P-wave duration at PET suggests possible downward rupture directivity during the Mw 6.7 earthquake. However, to test this possibility we first need to account for possible path effects such as wave diffractions along the high-velocity slab in which the earthquake occurred and site effects at the stations. We used waveforms from two nearby smaller earthquakes (Fig. 1; the 24 June 2013 Mw 4.3 sciencemag.org SCIENCE
